Method for developing latent electrostatic charge halftone images



Oct. 19, 1965 H. E. J. NEUGEBAUER 3,212,888

METHOD FOR DEVELOPING LATENT ELECTROSTATIC CHARGE HALFTONE IMAGES Filed June 12, 1961 3 Sheets-Sheet 1 F/G 2A @6669 6699 @999 @666 1 F bl/ --/o F/G. 2B

93%) 6mm) @(9 @(0 @em I --/o F/G. 2c

INVENTOR.

HANS E. J. NEUGEBAUER ATTORNEY Oct. 19, 1965 H. E. J. NEUGEBAUER 3,212,383

METHOD FOR DEVELOPING LATENT ELECTROSTATIC CHARGE HALFTONE IMAGES Filed June 12, 1961 3 Sheets-Sheet 2 a DQ BKD QCF fi Q v/ HANS E. J. NEUGEBAUER fQ k A T TORNE Y 1960 H. E. J. NEUGEBAUER 3,212,383

METHOD FOR DEVELOPING LATENT ELECTROSTATIC CHARGE HALFTONE IMAGES Filed June 12, 1961 3 Sheets-Sheet 3 zommzzmummmm 555 "235 FORM HALFTONE DEVELOP CHARGE CHARGE PLATE WITH RECHARGE TRANSFER XEROGRAPHIC PATTERN IN POSITIVE AND PLATE DEVELOPED PLATE IMAGE NEGATIVE IMAGE CONFIGURATION PARTICLES INVENTOR.

HANS E. J. NEUGEBAUER 8 QUQQ United States Patent 3,212,888 METHOD FOR DEVELOPING LATENT ELECTRO- STATIC CHARGE HALFTONE IMAGES Hans E. J. Neugehauer, Webster, N.Y., assignor to Xerox gorporation, Rochester, N.Y., a corporation of New ork Filed June 12, 1961, Ser. No. 116,429 4 Claims. (Cl. 96-1) This invention relates to Xerography and, more particularly, to improved methods, materials, and apparatus for xerographic development.

In the conventional forms of xerography an electrostatic image pattern is formed in response to a light pattern and the electrostatic pattern is then made visible through the selective deposition of pigmented material. The electrostatic image is generally formed through the combined action of light and an electric field on a xerographic plate which includes as its active element a layer of photoconductive insulating material. Development of the resulting electrostatic image pattern may be carried out by pouring or spraying various forms of electrostatically attractable coloring matter over the plate or by immersing the plate in various forms of suspensions of such materials. The resulting pattern of pigmented coloring material may be viewed on the plate or transferred to some other support. The basic xerographic processes mentioned above together with numerous variations are well known to the art through various patents and publications and through the widespread commercial availability and utilization of xerographic equipment. For these reasons the basic processes will not be described at length.

It is a general characteristic of most Xerographic development processes, in contrast to photographic development processes, that they tend to emphasize sharp edges or discontinuities in images. This characteristic is bene ficial in certain applications, particularly in the reproducton of fine printed matter or other so-called linecopy images where it leads to the production of images with dense black areas and clear white background areas. This characteristic, however, makes most xerographic development processes poorly suited for the reproduction of large solid areas or of continuous tone subjects such as photographs. This edge enhancement effect is particularly characteristic of the so-called cascade development process which is described, for example, in US. Patent 2,777,957 and US. Patent 2,836,725. In this process development is efiected by pouring over the surface of an electrostatic image bearing xerographic plate a mixture of micronsized pigmented resin particles known as toner and very much larger particles known as carrier. The triboelectric properties of the toner and carrier are so chosen that they acquire opposite electrical charges upon contact with each other and the carrier particles accordingly become coated with a layer of electrostatically attracted toner particles. As this developer material slides or cascades over the Xerographic plate, the toner particles are detached in areas of high electric field from the carrier particles because the attractive forces emanating from the xerographic plate overcome the attractive forces between the toner and the carrier and cause the toner particles to deposit on the xerographic plate. This development procedure is a very simple one and requires a minimum of apparatus and has become the most widely employed development method in commercial xerography. It is, however, particularly subject to the edge enhancement eifect mentioned previously and is therefore not Well suited for the reproduction of images with large solid areas.

Various xerographic development methods are known which do have the ability to reproduce solid areas and 3,212,888 Patented Oct. 19, 1965 in some cases continuous tone images as well. Generally, however, these methods are relatively complex and accordingly have limited commercial utility. Various modifications may also be made in the cascade development procedure to give it the capability of solid area reproduction and at least one of these, described in US. Patent 2,777,418, has achieved considerable commercial success. These methods nevertheless do not fully meet the requirements for simplicity of apparatus and reliability of operation together with dense images of high quality. A further method of improving solid area coverage involves the application of halftone techniques to convert solid image areas into a repetitive pattern of dots or lines, thereby rendering them more susceptible to development by simple methods such as cascade development. These methods do permit solid area coverage but at the expense of a great loss in image density since image areas intended to be black are reproduced as alternating regions of black and white having about equal area, thus providing gray rather than black areas. It is now disclosed, however, that by combining some of the principles of halftone reproduction with certain novel procedures and developer compositions it is possible to achieve solid area xerographic reproduction with high image densities while retaining, where desired, the simplicity and reliability of conventional cascade development. The invention is accordingly directed to novel development procedures and apparatus and to novel developer compositions for use in xerography. The various features and characteristics of the invention will be described in connection with the drawings in which:

FIG. 1 schematically represents the conventional xerographic process steps;

FIG. 2 schematically represents the properties of a coarse xerographic halftone pattern;

FIG. 3 schematically represents a fine Xerographic halftone pattern;

FIG. 4 schematically represents an excessively fine xerographic halftone pattern;

FIG. 5 represents a method of halftoning for positiveto-positive reproduction;

FIG. 6 represents a method of halftoning for negativeto-positive reproduction; and

FIG. 7 is a flow chart summarizing the invention.

FIG. 1 shows for illustrative purposes the most conventional xerographic process steps, although it will be appreciated that many variations and equivalents of the illustrated steps are known to the art and may be employed in conjunction with the present invention. FIG. 1A represents the electrostatic charging of a xerographic plate 10 which conventionally comprises a mechanical support member 11 upon which is coated a layer of photoconductive insulator 12. Application of an electrostatic charge to the surface of photoconductive insulating layer 12 is conventionally the first step in making a xerographic reproduction and may be carried out by moving corona charging device 13, connected to high voltage power supply 14, relatively to the xerographic plate 10. Corona charging apparatus is described in US. Patents 2,777,957

.and 2,836,725 and other methods of sensitizing are also known. The next step may be, as illustrated in FIG. 1B, the exposure of plate 10 to a pattern of light and shadow which is intended to be reproduced. This may be carried out by projecting an image onto plate 10 from enlarger 15 or by placing plate 10 in a camera or by other means. Exposure to a pattern of light and shadow causes a selective increase in the conductivity of photoconductive insulating layer 12 and a corresponding selective dissipation of the charge placed thereon in FIG. 1A. There is accordingly produced on xerographic plate 10 an electrostatic pattern Which is developable to make it visible.

FIG. 1C illustrates one method of accomplishing development. Plate is supported at an angle between horizontal and vertical and a developer composition 16 is poured over the plate. Developer 16 may be of the cascade type already referred to and the procedure of FIG. 1C causes a pattern of fine pigmented particles to deposit on the surface of plate 10. It will be appreciated that many methods, materials and apparatus for development are known in addition to that illustrated and have equal utility. image pattern from plate 10 to a more convenient support and this may be accomplished, as shown in FIG. ID, by placing a sheet of paper 17 or the like over plate 10 and moving plate 10 relative to a corona charging device which may be the same one illustrated in FIG. 1A. By this means the developer particles are electrostatically attracted from plate 10 to paper 17 and may be removed with paper 17 and may also be permanently affixed thereto by known methods. Other methods of transferring a developed image are known and some of these use adhesive materials rather than electrostatic forces to effect transfer.

Xerographic processes of the general type described above tend to enhance image discontinuities and to be poorly adapted for reproducing extensive black image areas or extensive areas of consistent density. FIG. 2 illustrates the reasons for this basic characteristic of xerography. FIG. 2A is a schematic cross section through a xerographic plate 10 showing a pattern of charges on the surface of the plate together with the associated electrostatic lines of force as they are believed to exist. The charges are illustrated as being positive charges because they are more commonly employed with commercially available xerographic plates, but the effects would be identical with negative charges. The illustrated pattern of charges corresponds to one that would be formed by exposing a charged plate to a pattern of light and shadow having a large black area, i.e., very large in relation to the thickness of photoconductive insulating layer 12 which typically ranges from a few microns to a few thousandths of an inch in thickness. The charged areas correspond to black areas of the pattern of light and shadow and the uncharged areas to light areas of the original pattern. Only the lines of force external to plate 10 are shown since these are the only ones effective in causing image development. It will be appreciated, however, that due to the proximity of the surface of photoconductive insulating layer 12 to support member 11 most of the lines of force actually travel through layer -12 rather than externally thereof. It can be seen from FIG. 2A that the lines of force concentrate around the edges of the charged areas, indicating strong fields in those regions and are widely spread apart in the center of the charged areas corresponding to a weak external electrostatic field in those areas.

Since it is these electrostatic fields which cause the attraction of charged developer particles it is possible to illustrate, as in FIGS. 2B and 2C, the effects of developing a charge pattern as shown in FIG. 2A with negatively or positively charged particles. FIG. 2B shows how negative toner particles 18 are attracted to the image of FIG. 2A and FIG. 20 shows the same for positive particles 18. Negatively charged particles are most generally used with a positively charged image and therefore FIG. 2B represents typical xerographic development. It can be seen that the particles deposit principally just inside the borders of large charged areas but do not deposit in the interior of such areas. Relatively small charged areas are completely developed as shown and as is generally known to occur, areas adjacent to charged areas are not developed because the direction of the electric field is such as to repel negatively charged particles. In FIG. 2C it can be seen that development is confined to areas adjacent to charged areas where the electric field is in a direction to attract positive particles to plate 10. The charged areas themselves, however, remain undeveloped because they It is usually desired to transfer the developed repel positive particles. Thus, FIG. 2 is representative of results generally obtained in xerography and illustrates and explains the effects of edge enhancement and lack of solid area coverage.

FIG. 3 is similar to FIG. 2 except that the large charged area is replaced by a pattern of small charged areas separated by small uncharged areas. Again, FIG. 3B represents development with negatively charged particles and FIG. 3C with positively charged particles. With the negatively charged particles all charged areas are fully developed and uncharged areas remain undeveloped. Thus the area corresponding to the large uniformly charged area of FIG. 2 is now developed as an alternating pattern of areas where toner particles 18 are present and where they are not. Thus the developed density in this area appears uniform when viewed from a distance but is low, since only about half the area of plate 10 in this region is covered by toner particles 18. FIG. 3 is thus illustrative of halftoning methods in xerography in which large solid charged areas are converted into a series of small separated charged areas. FIG. 3 is, of course, also representative of linecopy images or the like in which the black areas are inherently small. FIG. 3C represents development of this same image pattern with positively charged particles. Once again it is the areas adjacent to the charged areas which are developed rather than the charged areas themselves. It may be noted, however, that where a pattern of closely spaced charged areas is present, the pattern of particles deposited in FIG. 3C will scarcely be distinguishable from a distance from that illustrated in FIG. 3B, since each represents an alternating pattern of particles and no particles and therefore merely appears gray at a distance. It will also be noted in FIG. 3B that because the charged areas are all small in extent, the electric fields associated with them do not extend very far and thus particles having the same polarity of charge as these areas, as in FIG. 3C, deposit adjacent to these areas rather than at a distance therefrom. This is quite different from the situation illustrated in FIG. 2A or FIG. 2C where the fields associated with large charged areas extend a considerable distance away and cause deposition of like charged particles for a considerable distance as shown in FIG. 2C.

FIG. 4 is similar to FIG. 3 except that the image areas are represented by an alternating pat-tern of charged and uncharged areas so finely spaced that certain new effects appear. With commercially available xerographic plates and developer materials, these effects have been found to occur when the charge pattern has a fineness of about lines per inch although with other materials the critical fineness will be somewhat different. It will also be understood that the electric field pattern shown in FIG. 4 is speculative in character. The principal point to be noted in connection with FIG. 4 is that as the individual charged areas become closely spaced from each other the electric field pattern begins to resemble that shown in FIG. 2 where the charges are, in fact, uniformly distributed over large areas. Thus, there again appear extensive fringing fields surrounding the regions of charge, and where development is effected with particles of the same polarity as the charge on xerographic plate 10, particles again deposit at substantial distances from the charged areas.

The present invention involves the use and development of halftone type electrostatic images similar to those illustrated in FIG. 3. It will be appreciated, however, that such electrostatic images do not normally arise in xerography since exposure of a charged xerographic plate to a light pattern containing large solid areas gives rise to a charge pattern likewise containing large areas of uniform charge. Accordingly, it is necessary to provide special procedures for forming such charge patterns and two such methods are illustrated in FIG. 5 and FIG. 6. FIG. 5A shows the exposure of an already charged xerographic plate 16 to a halftone pattern of light and shadow.

In the illustrated embodiment this pattern of light and shadow is created by placing a halftone screen 20, i.e., a screen containing alternating opaque and transparent areas, adjacent to the surface of the charged xerographic plate and exposing it to uniform illumination. The same effect could be produced by using enlarger 15 of FIG. 1B or the like to project a similar halftone screen pattern onto plate 10. FIG. 5B represents the charge density, or potential, remaining on the surface of plate after exposure shown in FIG. 5A. It merely indicates that the originally applied charge remains in non-illuminated areas but is dissipated in illuminated areas. The next step, illustrated in FIG. 5C, is the exposure of plate 10 to an arbitrary image pattern of light and shadow which is represented in this figure as a step wedge transparency 21. Again, it will be noted that this exposure step could equally well be carried out with an enlarger, camera, or other means. FIG. 5D represents the charge remaining on the surface of plate 10 after the exposures of FIGS. 5A and 5C. It will be appreciated that exactly the same charge pattern will be formed if the exposure step of FIG. 5C is performed before instead of after that of FIG. 5A. In accordance with this procedure there is formed an electrostatic charge pattern consisting of areas of Zero or low charge alternating with areas of higher charge, the charge of which is inversely related to the image exposure received. By referring back to FIG. 3, it will be seen that during development a pattern of toner particles will be formed in those regions receiving less illumination during the exposure step of FIG. 5C and that substantially no toner will deposit in areas receiving maximum illumination. Furthermore, this relation is true regardless of the polarity of charge originally applied to plate 10 or to the polarity of toner during the development step.

FIG. 6 shows a different method of obtaining a halftone effect in xerography. In this embodiment halftone screen 20 and image transparency 21 are placed together and plate 10 is exposed through the combination of elements 20 and 21. Again it is to be noted that exposure could also be carried out with projectors, cameras, or the like as well as by the contact exposure method shown. FIG. 6A shows the exposure step as such and FIG. 6B shows the charge remaining on plate 10 after exposure. It can be seen that in regions corresponding to the black portions of transparency 21 that plate 10 retains a uniform potential whereas in regions corresponding to transparent areas of transparency 21 the charge pattern consists of alternating areas of maximum charge and areas whose charge is related inversely to the exposure received through transparency 21. In accordance with the development principles already described in connection with FIG. 3, the areas of plate 10 corresponding to the black areas of transparency 21 will receive substantially no toner particles during a conventional development step whereas the regions of plate 10 corresponding to transparent areas of transparency 21 will be developed with a pattern of toner particles. This will be true regardless of the polarity of charge on plate 10 or on the toner particles. This relationship between exposure and development is exactly the opposite of that produced by FIG. 5. The method of FIG. 5 leads to positive-to-positive reproductions wherein areas corresponding to the black image areas receive the maximum toner deposition while the method of FIG. 6 leads to a negative-to-positive form of reproduction where areas corresponding to black areas of the original receive the least toner deposition. The methods illustrated in FIGS. 5 and 6 are distinguished by the fact that they do not depend on any unusual forms of processing apparatus or unusual forms of xerographic plate 10. However, such apparatus and such plates do exist and can lead to the results achieved by the methods of FIGS. 5 and 6 in a simpler fashion without departing from the principles inherent in the methods of these figures and without departing from the spirit of the invention. Illustrative examples of other methods of forming charged halftone patterns will be described later.

The next step in carrying out the present invention, following the steps of either FIG. 5 or FIG. 6 or their equivalents, is to develop the electrostatic charge pattern on plate 10 to make it visible. In accordance with the present invention this is carried out by depositing on plate 10 particles having both positive and negative electrical polarities. Positive and negative developer compositions are both well known in the art and available commercially. In the cascade development process the charge on the toner particles is determined by the triboelectric relations between the toner and carrier particles. Thus the polarity of toner charge can be controlled by varying either the composition of the toner or the carrier or both. For the purposes of the present invention it does not matter how the polarity is controlled. Interesting effects can be achieved by using different toners of contrasting color, but it is generally preferred that all toner particles be of substantially the same appearance. As a practical matter it has been found most effective to use a single toner material and to control the polarity of charge on that toner by employing different carrier materials. Suitable materials are described in US. Patents 2,618,551, 2,940,934, and also 2,618,552, 2,638,416, 2,659,678, 2,753,308, 2,788,288, and 2,892,794 and suitable cascade development materials are also available from Xerox Corporation, Rochester, New York, under the names Xerox Type 10 developer, Xerox reversal developer Type 10, Copyflo D1 developer, and Copyflo N1 developer.

Development may be carried out by first applying a developer with positive toner particles to plate 10 and then a developer with negative toner particles or vice versa since the same results are achieved either way. The result of such a development procedure is that the negative particles deposit generally in areas indicated in FIG. 33 while the positive particles deposit in areas indicated in FIG. 3C without the two polarities substantially mixing or interfering with each other. In accordance with FIGS. 3B and 3C, it can be seen that in areas of plate 10 carrying a halftone charge pattern that the negative and positive particles deposit in substantially mutually exclusive areas and that the two polarities together occupy substantially the entire areas in which a halftone charge pattern appears. The exception to this principle is the line which forms the actual boundary of the individual areas of charge. This line represents an area where the electrostatic field at plate 10 is parallel to the surface of the plate and provides neither an attractive nor a repulsive force towards the plate for particles of either polarity. Accordingly, the individual charged areas are developed with particles of one polarity; the surrounding areas are developed with particles of the other polarity and the two groups of particles are separated by a minute line of no particles. Since nearly all areas of the image are now covered with particles a high image density and :at the same time solid area coverage has been achieved.

It .is not necessary to employ two separate development steps to achieve these results since it has also been found that a single development step is effective when the mixture of positive and negative toner particles are simultaneously presented to plate 10. This effect can be accomplished by using a mixture of two different types of toner particles but can more conveniently be carried out by using a single type of toner material combined with two different carrier materials such that the toner particles adherent on one type of carrier have a positive charge and the toner particles adherent on the other type of carrier have a negative charge. Such a developer composition can be formed, for example, by mixing Xerox Type 10 developer with Xerox reversal developer Type 10. These developers use a common toner but different carriers to achieve negative and positive toner polarities respectively. Another suitable composition comprises a mixture of Copyflo P1 and Copyflo N1 developers, which also use a common toner component. Equal amounts of the two components may be employed, but the ratio is not at all critical. The above materials are available from Xerox Corporation, Rochester, New York. Such a developer mixture has the same physical properties as ordinary xerographic cascade developer and may be employed in any form of xerographic apparatus which has hitherto used conventional xerographic cascade developer materials. Various development methods other than cascade are also known to the art and may be employed herein. With each such method it is also possible by known means to provide development particles of either positive or negative polarity.

It has been found that not only has solid area coverage with high density been achieved with the method of the present invention but also that continuous tone reproduction is achieved. In other words, the developed image density is not only uniform throughout large black areas but the density varies substantially proportionally with the density of the original subject matter. The mechanisms responsible for this effect are not fully understood but high quality continuous tone reproduction is consistently obtained with the present invention.

It is necessary in carrying out the present invention to form an electrostatic charge pattern consisting of alternating areas of charge and no charge but these areas need not necessarily be in the form of dots. It is presently believed that the most desirable results are obtained by forming a charge pattern in the form of alternating lines of charge and no charge and this may be accomplished by using a screen 20 in the form of a grid of alternating transparent and opaque line areas. It has been noted that development in accordance with the present invention leaves a narrow undeveloped line around each individual element of charge on plate 10. It is believed that best results are obtained with a line screen rather than :a dot screen because for a given fineness of screen the undeveloped edges occupy a smaller fraction of the developed image. While the term halftone has been used to describe the screen patterns employed in the present invention, that term is intended to encompass line screens and line patterns as well as other forms of screens in addition to the more common dot pattern screen.

Fineness, or the number of lines per inch of the screen pattern is obviously important in achieving the best results obtainable with the present invention. When the screen pattern is excessively coarse the resulting images exhibit a readily visible and objectionable pattern and image quality is also degraded by the fact that the toner having the same polarity as the electrostatic charge pattern deposits in extensive undesired areas. This effect can be understood from an examination of FIG. 2C. If on the other hand, the screen pattern is too fine it has been found that fine image details which should be reproduced as clear areas are instead filled in with deposited toner. This effect is actually similar to the undesired effect noted in connection with excessively coarse screen patterns and is believed due to a similar cause as seen from FIG. 4C. With commercial developer materials and commercial vitreous selenium xerographic plates having a photoconductive insulating layer on the order of about 1 to about 2 thousandths inch in thickness, this last efiect becomes apparent with screens having approximately 90 lines per inch. This critical figure obviously Will be somewhat different for other types of plates and other developers. The preferred fineness of the screen pattern is one which is just coarse enough to avoid the fill-in effect noted with excessively fine screens. For the xerographic materials in commercial use the optimum screen is therefore about 75 to about 80 lines per inch.

The halftoning method of FIG. 5 represents a method of forming a pattern of uncharged areas in an otherwise conventional xerographic charge pattern. Other methods are known which achieve the same results and may be substituted in this invention for the illustrated method of FIG. 5. Thus, plate 10 may be charged prior to image exposure by the method of FIG. 1A with a perforated screen positioned on or closely adjacent to the plate. The plate is thus charged in a screen pattern and this is effectively equivalent to uniformly charging the plate and then discharging it in a screen pattern. Plate 10 can also be charged in a screen pattern by contacting it with an embossed semi-conductive roller or platen maintained at a high potential or a uniformly charged plate may be selectively discharged by contact with an embossed and grounded element. It is also possible to manufacture xerographic plates which are not uniformly adapted to maintain an electrostatic charge in darkness but instead contain a pattern of areas which are not capable of retaining an electrostatic charge under any condition.

The form of charge pattern created by the manipulations of FIG. 6 may also be simulated through the use of a xerographic plate having localized areas which are not photosensitive and which will retain rather than dissipate any charge applied thereto. Such a plate may be formed by printing a pattern of insulating plastic on top of the photoconductive insulating layer 12. It is also possible to manufacture a plate 10 including a transparent element as support member 11 and support member 11 may then contain a halftone screen 20 as an integral element. With such a plate the electrostatic image of FIG. 5 can be duplicated by applying the image exposure to the photoconductive insulating layer of the plate and applying uniform illumination to the transparent support member 11, while the electrostatic image of FIG. 6 may be simulated by making the image exposure through the transparent support member.

The developed image formed in accordance with the present invention provides a development of solid areas which is highly uniform in visual appearance yet consists of alternating areas of positively and negatively charged toner particles. Such an image may be transferred from xerographic plate 10 to another support by adhesive methods but cannot be transferred by electrostatic method illustrated in FIG. 1D inasmuch as the method of FIG. ID will result in the transfer to transfer paper 17 of only half the image since one polarity of toner will be attracted to the paper while the other will be repelled. However, the electrostatic transfer method of FIG. 1D is the generally preferred transfer method because it is usable with ordinary paper as opposed to the specially coated transfer materials normally required with other processes. To accomplish transfer electrostatically a developed image according to the present invention can be transferred by subjecting it first to the charging process shown in FIG. 1A and then to the transfer process of FIG. 1D. The procedure of FIG. 1A applies a single polarity of charge to the developed xerographic plate 10 and any toner particles 18 thereon. After this charging step the developed image consists of one electrical polarity of particles only and these particles may then be transferred to a sheet of paper or the like by the procedure of FIG. 1D. It will be noted, however, that the polarity of power supply 14 must be reversed before carrying out the final transfer step in order that the toner particles 18 be attracted to paper 17 rather than repelled herefrom. Such control is present on commercial xerographic equipment again indicating that this invention can be practised on existing structures used in this field. After toner particles 18 have been transferred to the sheet of paper or the like, they may be permanently affixed to the paper by any of the known methods for that purpose.

The essential process steps for carrying out the present invention are summarized in a flow sheet in FIG. 8. However, the last two steps may obviously be omitted where it is satisfactory to view the developed image on the xerographic plate itself.

Methods are also known whereby a halftone charge pattern in image configuration can be formed without a separate and distinct charging step and such methods are of course also useful in the present invention.

The invention has been described largely in terms of specific embodiments, materials and manipulations but this has been for illustrative purposes only. Thus, charging of a xerographic plate can be carried out by methods other than those shown and described and electrostatic charge patterns can be formed in response to a light pattern without the necessity of separate and distinct charging and exposing steps. Furthermore, methods are also known for forming charge patterns of a halftone character on materials other than photoconductive insulators and by other than optical methods. While the invention has largely been described in terms of so-called cascade development because this is the simplest and most widely used method, the invention may also be employed with any electrostatic image development method in which it is desired to effect an improvement in solid area or halftone reproduction capabilities. Xerographic plates may be made of various materials, may be flexible as well as rigid, or may have various shapes other than that illustrated. Any or all xerographic process steps may be carried out on various forms of automatic or semi-automatic equipment. These and other variations in the invention will readily occur to those skilled in the art and are intended to be encompassed within the invention.

What is claimed is:

1. The method of forming a positive-to-positive xerographic reproduction of a pattern of light and shadow comprising uniformly electrostatically charging a xerographic plate, exposing the plate to said pattern of light and shadow and at least after charging and before development thereof exposing said plate to a halftone pat tern of light whereby a repetitive pattern of discharged areas is formed on said plate, developing said plate by flowing over its surface a mixture of a single species of finely divided pigmented toner material and two species of relatively larger carrier particles each species of carrier being physically similar but having a triboelectric relationship of opposite polarity to said toner, charging all toner particles on said xerographic plate to a common electrical polarity, and electrostatically transferring said toner particles from said xerographic plate to a separate support element.

2. The method of forming a negative-to-positive xerographic reproduction of a pattern of light and shadow comprising uniformly electrostatically charging a xerographic plate, exposing the plate to said pattern of light and shadow through a halftone screen, developing said plate by flowing over its surface a mixture of a single species of finely divided pigmented toner material and two species of relatively larger carrier particles each species of carrier being physically similar but having a triboelectric relationship of opposite polarity to said toner, charging all toner particles on said xerographic plate to a common electrical polarity, and electrostatically transferring said toner particles from said xerographic plate to a separate support element.

3. The method of developing a xerographic plate bearing a halftone latent charge pattern comprising flowing over said plate a developer composition including a single species of finely divided toner particles and two different species of substantially larger and physically similar carrier particles, said toner being adherent upon and positively charged by one of said carrier species and being adherent upon and negatively charged by the other of said carrier species.

4. The method of forming a xerographic reproduction of a pattern of light and shadow comprising forming on a xerographic plate a screen pattern consisting of two alternating sets of lines of electric charge, one of said sets of lines being of fixed charge density, the other of said sets having a charge density which varies in relation to the brightness of said pattern of light and shadow, developing the lines of charge by flowing over said plate a developer composition including a single species of finely divided toner particles and two different species of substantially larger and physically similar carrier particles, said toner being adherent upon and positively charged by one of said carrier species and being adherent upon and negatively charged by the other of said carrier species, the coarseness of said screen pattern being just suflicient to avoid the deposition of toner in non-image areas of said pattern of light and shadow.

References Cited by the Examiner UNITED STATES PATENTS 2,297,691 10/42 Carlson 96-1 2,576,047 11/51 Schafit'ert 96-1 2,598,732 6/52 Walkup 96-1 2,808,328 10/57 Jacob 961 2,811,465 10/57 Greig 96-1 2,907,674 10/59 Metcalfe et al. 961 2,940,847 6/ Kaprelian.

2,965,573 12/60 Gundlach 25262.1 3,010,842 11/61 Ricker 25262.1 3,013,890 12/61 Bixby 25262.1 3,045,644 7/62 Schwertz.

3,078,231 2/63 Metcalfe et al. 96--1 3,104,169 9/63 Metcalfe et al. 961

NORMAN G. TORCHIN, Primary Examiner.

HAROLD N. BURSTEIN, Examiner. 

1. THE METHOD OF FORMING A POSITIVE-TO-POSITIVE XEROGRAPHIC REPRODUCTION OF A PATTERN OF LIGHT AND SHADOW COMPRISING UNIFORMLY ELECTROSTATICALLY CHARGING A XEROGRAPHIC PLATE, EXPOSING THE PLATE SAID PATTERN OF LIGHT AND SHADOW, AND AT LEAST AFTER CHARGING AND BEFORE DEVELOPMENT THEREOF EXPOSING SAID PLATE TO A HALFTONE PATTERN OF LIGHT WHEREBY A REPETIVE PATTERN OF DISCHARGED AREAS IS FORMED ON SAID PLATE, DEVELOPING SAID PLATE BY FLOWING OVER ITS SURFACE A MIXTURE OF A SINGLE SPECIES OF FINELY DIVIDED PIGMENTED TONER MATERIAL AND TWO SPECIES OF RELATIVELY LARGER CARRIER PARTICLES EACH SPECIES OF CARRIER BEING PHYSICALLY SIMILAR BUT HAVING A TRIBOELECTRIC RELATIONSHIP OF OPPOSITE POLARITY TO SAID TONER, CHARGING ALL TONER PARTICLES ON SAID XEROGRAPHIC PLATE TO A COMMON ELECTRICAL POLARITY, AND ELECTROSTATICALLY TRANSFERRING SAID TONER PARTICLES FROM SAID XEROGRAPHIC PLATE TO A SEPARATE SUPPORT ELEMENT. 